RetroArch even supports up to 16 ports to map controls to (I understand this is for games in which the controls are simple but you CAN map 16 ports theoretically)
I agree that commercially there are not that many games that have more than 4 players (but I would argue this is an artifact of the physical arcade space which with Parsec and wireless controllers goes away). However there are several modules that would benefit from 5-8 players such as the...
Thanks for the clarification on the "Plus"! I see... so if more than 4 players are utilized it would require a lot of custom code to consider the extra 2 players (ensuring all of the logic in the classes update attributes for those extra 2 players). For the hud/UI/UX I would imagine, copying...